The present invention relates generally to optical infrared spectrometer systems, and more particularly, to an optical infrared spectrometer system that employs an entrance slit spaced telecentrically by locating the aperture stop at the back focal point of the collimating optics.
Prior art relating to the present invention includes dispersive optical infrared spectrometers that use for example, either prisms or gratings. Typically, a dispersive optical system has a detector and an imaging slit onto which the detector is focused, and is designed to operate within the infrared region of the optical spectrum. Because all warm objects emit infrared radiation, the slit body is cooled to minimize self emitted radiation which is seen by the detector as background noise. Cooling requires that the slit be placed in a vacuum that is connected to a cryogen source, both of which features are expensive.
Prior systems typically cool both the focal plane (detector) and the slit. In some systems, two vacuum dewars (double walled vacuum container) or one complex dewar incorporating the slit and the focal plane have typically been used. In other systems, one large or two smaller cryogenic coolers have been used to cool the slit and the focal plane. But, cryogenic systems that cool the entire spectrometer or, cool just the slit body and the detector, are very bulky. Cryogenic cooling must be accomplished by immersing the dewar in a large vat of liquid cryogen, such as, liquid nitrogen, or by attachment of a large, powerful mechanism such as a Stirling cycle cryogenic refrigerator. However this approach incurs severe penalties in terms of high cost, large size, large weight, high electric power requirements (for the Stirling mechanism) and maintainability requirements (supplying and filling liquid cryogen in field conditions). Furthermore, special materials and designs have also been required to maintain system alignment from room temperature down to operational temperature.
Some prior systems have used a low reflectivity, high emissivity, flat slit surround or body that introduces a small percentage of the radiation from the cavity wall. However, all of the in-field radiation from the slit body is imaged onto the focal plane detector. Another prior art system uses a high reflectivity, low emissivity flat slit body that reduces the self emission from the slit body and increases the reflection contribution from the cavity. These two embodiments require that the slit body or the cavity walls be cooled to minimize background effects. Instrumental thermal background is a severe problem in infrared spectrometers. It produces increased noise, decreased dynamic range, and may decrease radiometric accuracy. These effects are so undesirable that infrared spectrometers are often cooled to cryogenic temperatures to substantially reduce the thermal background. These issues render the use of sensitive infrared spectrometers for observations of the earth from airborne or satellite platforms difficult. A solution to this problem is described in U.S. Pat. No. 5,534,700. Here the slit is configured as a highly reflective, low emissivity optic (generally a toroid for most spectrometer designs) so that self thermal emission from the slit body would be nil and that thermal emission from the spectrometer cavity reflected from the slit body would impinge outside the active detector area. The need for cryogenic cooling of the entire spectrometer is eliminated, since the instrumental thermal background is substantially reduced. It is noted that self thermal emission from the slit body is nil and thermal emission from the spectrometer cavity reflected from the slit body impinges outside the active detector area. However a flat reflective slit body is far easier to fabricate.
The design in U.S. Pat. No. 5,434,700 thus does not require a toroidal slit body, which is extremely difficult and costly to fabricate to the required tolerances. If the spectrometer covers a very large field of view, as is highly desirable for most imaging infrared spectrometers, the required toroidal surface may be so steep as to be impossible to fabricate with the prevailing state of the art. The need for the toroidal slit body may be obviated if, in accordance with the invention, a spectrometer design is used that (a) makes the entrance slit space telecentric by locating the aperture stop at the back focus of the collimating optics, and (b) places the dispersive element (a grating or prism) at or close to the aperture stop. In so doing, all the chief rays in both the spatial and spectral fields are oriented parallel to the optical axis.
In sum, the system disclosed in U.S. 5,534,700 requires a toroidal slit body, which is extremely difficult and costly to fabricate to the required tolerances. If the spectrometer covers a very large field of view, as is highly desirable for most imaging infrared spectrometers, the required torroidal surface may be so steep as to be impossible to fabricate with the prevailing state of the art.